In certain video disc systems, video information is recorded by means of geometric variations in the bottom of a smooth spiral groove on the surface of a disc record. The disc surface includes a coating of conductive material which is preferably covered with a thin deposit of dielectric material. A signal pickup engages the spiral groove and includes a conductive surface which, together with the conductive coating and the dielectric deposit of the disc record, form a capacitor. When the disc record is rotated, an edge of the conductive surface of the signal pickup, while riding in the disc record groove, recovers capacitive variations due to the geometric variations in the bottom of the spiral groove. The capacitive variations, indicative of the prerecorded video information (e.g., in the NTSC format), are applied to a suitable signal processing circuit and electrical signals obtained therefrom are then coupled to a conventional television receiver for reproduction. The variable capacitor concept, as applied to video disc systems, is described in detail in the U.S. Pat. No. 3,842,194, issued to J. K. Clemens, on Oct. 15, 1974, and entitled "INFORMATION RECORDS AND RECORDING/PLAYBACK SYSTEMS THEREFOR".
In the video disc systems of the aforementioned Clemens' type, it has been recognized that a predetermined speed relationship must be maintained between the disc record and the signal pickup to obtain accurate reproduction of the prerecorded signals. The predetermined speed and specified tolerance limits are also necessary to assure that the recovered horizontal and vertical synchronizing information is stable and within the lockup range of the deflection circuits of the television receiver. Moreover, when the prerecorded information is a color television signal with chrominance information recorded as a modulated carrier signal, the recovered signal must be stable and within the lockup range of the color processing circuits of the playback system in order to minimize color phase distortion.
The pickup/record speed errors could be minimized by employing precision components and design. However, this would be expensive and further, the wear and tear of the components during operation would require continuous readjustments. Consequently, it is desirable to provide an inexpensive system for automatically maintaining the rotation of the turntable at the predetermined speed. A speed control system is disclosed in the copending U.S. patent application, Ser. No. 478,653, filed June 12, 1974, for C. D. Boltz, entitled "TIMING ERROR DETECTING AND SPEED CONTROL SYSTEM", and now U.S. Pat. No. 3,940,556. The Boltz system uses the timing information included in the prerecorded signal developed at the output of the signal pickup to generate an error correction signal for maintaining the turntable rotation at the predetermined speed. It is noted that, when the system is initially switched on, the signals are absent until the signal pickup is properly riding in the information carrying groove and relative motion is established between the disc record and the signal pickup. Due to the absence of timing information during this interim period, the speed control system has no way of knowing the speed error and therefore the control system response during this interim start-up period must be arbitrary. Besides initial start-up period, the arbitrary response due to absence of signals at the output of the signal pickup could occur under certain other conditions: for example, signal dropouts on the disc record, interval between disc record changes, etc., to name a few. The present invention is advantageous in that the turntable speed control system does not depend upon the signal developed at the output of the signal pickup for the development of the error correction signal.
In the past, some speed control systems have employed frequency discrimination principle for generating the speed error correction signal. In these systems, the error correction signal is a function of the speed error, or in other words, frequency differential between the output signal and the reference signal. Therefore, the error correction signal must be zero or arbitrary when there is no speed error (i.e., frequency differential). Due to the necessity of a speed error for development of the error correction signal, such systems normally have built-in residual error. The present invention employs phase error detection principle for generating the speed error correction signal. In these systems, the error correction signal is a function of phase (i.e., integral of frequency) differential between the output signal and the reference signal. Therefore, the error correction signal (a function of phase difference) could be developed without any speed error (i.e., frequency differential). Typically, the phase detection speed control systems are relatively free from built-in residual error.